The present invention relates to a twin-cylinder circular knitting machine for manufacturing socks and stockings with knitted fabric tensioning device.
Twin-cylinder circular knitting machines are known which have a device for applying tension to the knitted fabric. The device comprises an element for retaining the initial portion of the fabric which is arranged inside the lower needle cylinder proximate to the region where the knitting is formed, and a fluid-activated piston provided with a hollow stem which is open at its ends and can slide inside a cylindrical chamber defined coaxially in the upper needle cylinder.
The retention element faces the lower end of the stem of the piston, and its dimensions are such as to allow the descent of the stem into the lower needle cylinder, passing around said retention element so as to apply tension to the fabric which extends from the retention region to the region of the machine where knitting is formed.
The upper end of the stem can be connected to an aspiration device which aspirates the fabric at the end of the knitting and turns it inside out.
The descent of the piston in order to apply tension to the fabric being formed is obtained by gravity by providing appropriate piston weighting elements, whereas the ascent of the piston is caused by a pressurized fluid which is fed into the cylindrical chamber.
These known types of twin-cylinder machines with device for applying tension to the fabric have some problems.
Due to the fact that the piston stem descends by gravity, damage to the fabric may occur due to excessive traction stresses, particularly in the initial portion of the fabric, when the lower end of the stem makes contact with the fabric and the weight of the piston, the stem and the weighting elements bears on a short length of fabric.
Furthermore, since the stem of the piston is rigidly associated with the upper needle cylinder in rotation about its axis, while the cylindrical chamber in which it slides is defined by a tubular body which is fixed to the supporting structure of the upper needle cylinder, there occurs rapid wear of the gaskets interposed between the piston and the tubular body in which it slides.
The tension application device is furthermore provided with an engagement device which is mounted on the supporting structure of the upper needle cylinder and can engage the piston in order to keep it in a raised position when the application of tension to the fabric is not required. The use of the engagement device which must engage the piston, which rotates together with the upper needle cylinder, complicates the connection between said two elements.